This invention relates to a process for the preparation of aromatic cyanates. More specifically, it relates to the preparation of aromatic cyanates using cyanogen chloride generated in situ.
Aromatic cyanates are useful in the preparation of polytriazines. Polytriazines are useful as cure in place resins and can be fabricated in the form of shaped articles, where thermostability, chemical inertness, and solvent resistance is desirable or required.
There are several known methods for the preparation of cyanogen chloride using high temperature vapor phase reactions of chlorine, hydrogen cyanide, or oxygen, hydrogen chloride and hydrogen cyanide, but the product prepared has to be carefully separated from unreacted starting materials using complicated purification procedures. It is known to prepare cyanogen chloride by bubbling chlorine gas into a mixture of zinc sulfate and potassium cyanide in an aqueous medium. See Handbook of Preparative Inorganic Chemistry, 1, 662 (1963), Academic Press, N.Y. The presence of zinc sulfate rules out the possibility of using this crude solution for polycyanate synthesis as it is known that zinc salts are effective catalysts for the cyclotrimerization of aryl cyanates. It is further known to prepare cyanogen chloride by adding chlorine gas to a slurry of solid sodium cyanide in carbon tetrachloride and acidic acid. Aryl cyanates are very reactive towards nucleophiles such as carboxylic acids, therefore, this procedure is not suitable when the cyanogen chloride is to be used in situ to further prepare aryl cyanates. See Inorganic Synthesis, 2, 90 (1946).
Martin et al., Organic Synthesis, 61, 35, disclose a process which involves the addition of an aqueous solution of sodium cyanide to a mixture of bromine and water to prepare cyanogen bromide and sodium chloride. A solution of a phenolic compound in carbon tetrachloride is then added followed by triethylamine. The removal of the solvent from the organic layer and distillation of the residue gives aryl cyanate.
What is needed is a process for the in situ preparation of cyanogen chloride wherein the cyanogen chloride can thereafter be used to prepare stable aromatic cyanates.